There are two players with Decay on the default teams:
Nurgle Rotters, who don't have Regeneration, and Tomb Guardians, with Decay and Regeneration.
Both teams can't have an Apothecary, but they can hire an Igor (100,000 gps) for one match only, who allows to re-roll failed Regeneration rolls.
Since the Rotter has no Regeneration, Igor can't be used on him.
The Tomb Guardian, when suffering a Casualty, will roll twice on the Casualty table, and then he can try to regenerate. If Regeneration works, both injuries are healed, if Regeneration fails, the Igor can be used to re-roll the Regeneration.